Anemia – Part 5 – Sickle Cell Screening (Hemoglobin S) Sickle Cell Anemia, Sickle cell preparation
- The screening test can be done on the EDTA blood.
- Electrophoresis is more accurate, should be done in the positive sickle cell screening test.
Purpose of the test (Indication)
- To diagnose sickle cell anemia (Sickle cell disease or trait).
- Please note if the patients have a blood transfusion in the last 3 to 4 days which will give the negative test.
- Polycythemia gives the false-positive test.
- Drugs that give false negative result is a phenothiazine.
- Infants less than the age of 3 months may give a false negative result.
- Definition: Sickle cell anemia is an autosomal disorder caused by the substitution of the amino acid Valine for Glutamine at the 6th position of the β-hemoglobin chain, which will give rise to the production of Hb-S.
- Epidemiology: There are 8 to 10% of African Americans and a lesser percentage of patients seen in the eastern Mediterranian, India, and Saudi Arabia.
- Sickle cell anemia represents the most common type of severe hemoglobinopathy, with an estimated prevalence in the united states of 1 in 375 African – American live births.
- There are two major clinical features of the Sickle cell anemia are:
- Chronic hemolysis.
- Acute episodic vaso-occlusive crises that may cause organ failure accounts for the mortality and morbidity associated with the disease.
- Sickle cell anemia is caused by the abnormality of Hb.
- It is a group of hemoglobin disorders in which the sickle β-globin gene is inherited.
- Sickle cell disease exists in tow forms:
- Homologous HbS (HbSS), sickle cell anemia.
- Heterologous HbS (HbSC), sickle cell disease.
- Sickle β – thalassemia.
- Sickle cell anemia is due to the genetic homozygous defect and is caused by the presence of HbS instead of HbA.
- The protein which carries the oxygen from the lungs to the tissues in the RBC Hb is an abnormal Hb called Hemoglobin S (Hb S).
- The life span of RBC with HbS is short than the normal RBCs.
- Also, these abnormal RBCs lost their normal shape and become rigid, deformed, and become sickle or crescent-like structure.
- When HbS becomes deoxygenated, it tends to change them into the sickle shape, which is an agriculture tool with a semicircular shape.
- Sickle cells in the circulation increase the viscosity of the blood, which will slow the circulation, thereby increasing the time of exposure to a hypoxic condition particularly in the small vasculature of the spleen.
- Repeated sickling of the RBCs ultimately damages the membrane permanently.
- These cells can not squeeze freely through the capillaries that may block the blood vessel (microvascular tree).
- The area of that organs which is deprived of blood supply due to blockage by these sickle cell damage the tissue or organs.
- Homozygous sickle cell disease is associated with increased morbidity and mortality.
- Heterozygous sickle cell disease has less mortality.
- Routine peripheral blood smears do not show these sickle-shaped cells unless there is hypoxemia.
- Normal = Negative
- In Sickle disease, there is 80 to 100% of Hb S (Homozygous Sickle cell disease).
- 20 to 40% Hb S present in the sickle cell trait.
- A definite diagnosis of Sickle cell disease is confirmed by electrophoresis.
Signs and symptoms:
- These patients have severe hemolytic anemia.
- There is chronic hemolytic anemia and vaso-occlusion, resulting in ischemic tissue injury.
- There may be vascular occlusive crises due to blockage of the blood vessels.
- All the organs are at risk for the injury due to vascular obstruction and the organs at greater risk are the spleen, kidney, and bone marrow.
- The eye and head of the femur are the targets for ischemic injury because of the limited blood supply to these organs.
- There may be cerebral vascular accidents.
- In older patients organ damage is evident.
- There may acute chest syndrome.
- HbSS patients present with severe chronic hemolytic anemia in early life and the Hb level is in the range of 6 to 8 g/dL.
- These patients may be mildly jaundiced.
- These patients may develop sickle cell crises:
- Aplastic crises may be due to infection like parvoviruses, which cause temporary suppression of erythropoiesis.
- Hemolytic crises reflect the acute attack with resulting in the fall in Hb, Hct, increased retics, and jaundice.
- Pain (vaso-occlusive) is the hallmark of sickle cell anemia. The pain is very severe and caused due to the occlusion of the blood vessel. The painful crises may last for 4 to 6 days and sometimes persists for weeks.
Sickle cell trait (Hb A/S).
- Hb A = 60 to 80%.
- Hb S = 20 to 40%.
- Hb F = small amount (normal)
- HbA2 = varies
- The molecular structure = α2 βA βS
- These patients do not show signs but can inherit the gene for children.
- Sickle cell trait may lead to :
- Renal papillary necrosis.
- Increased risk of pulmonary embolism.
- Signs and symptoms:
- This patient can develop severe hematuria.
- It can have a fixed urine specific gravity of 1.010 due to renal damage.
- Some of the patients are normal.
- Diagnosis :
- Normal RBCs.
- Normal Hb.
- Normal Hct.
- Normal reticulocytes.
- Sickle screening is positive.
Sickle cell anemia (Homozygous state, HbS disease):
- HbA = absent. (0%)
- HbS = 80 to 100%.
- HbF = variable
- HbA2 = variable
- The molecular structure = α2 βS βS
- There are all the clinical signs of the disease.
- HbS in combination with thalassemia or HbS with HbC may be seen (S/C).
Diagnosis (Lab tests advised are):
- The affected child shows limitation to the exercise, shortness of breath, tachycardia, frequent severe infection, and an episode of very painful dactylitis (painful inflammation of fingers or toes).
- Complete blood count:
- Low RBC count.
- Low Hb.
- Low Hct.
- MCV is normal.
- MCHC is normal.
- WBCs and platelets are usually high.
- Increased reticulocytes.
- Peripheral blood smear show inclusion like Howel-Jolly bodies and Pappenheimer bodies.
- Sickle cells test to see the sickle cells.
- Hemoglobin electrophoresis.
- Measurement of Hb A2 and Hb F.
- Bone marrow shows hyperplasia.
Method to find Sickle cell
- Sickle cell preparation Add sodium metabisulphite 2% to EDTA blood.
- Examine the slide immediately and then after one hour.
- If there are 25% or more HbS is present in the RBCs then (there are crescent or) sickle-like cell = Test is positive.
- The second method is that put a drop of blood on the slide and seals it with paraffin.
- Leave the slide for some time and keep on seeing the slide for the presence of a sickle cell.
- The third method Add dithionite to the blood.
- Hb S will precipitate.
- A negative test indicates that the patient has no or very little <10% HbS.
Please see more details in part 1.